1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to a secondary battery having a current interrupt device (CID) between the bottom of a case and a negative current collecting plate.
2. Description of the Related Art
As generally known, a secondary battery can be recharged and discharged, unlike a primary battery. Secondary batteries may be classified into low-capacity batteries and high-capacity batteries.
For example, a low-capacity secondary battery includes unit batteries, and is mainly used for small portable electronic devices, such as cellular phones, laptop computers, and camcorders, whereas high-capacity batteries are used as the power supply for driving motors in hybrid electric vehicles and the like. A high-capacity secondary battery forms a battery module by connecting a plurality of unit batteries in the form of a pack, and is used as the power supply for driving motors in hybrid electric vehicles and the like.
Each of the unit batteries includes an electrode assembly including a positive electrode, a separator, and a negative electrode, a case for accommodating the electrode assembly, and a cap assembly coupled with the case to seal the case and having an electrode terminal electrically connected with the electrode assembly.
For example, in a cylindrical secondary battery, the positive and negative electrodes in the electrode assembly respectively include non-coating portions on which an active material is not coated, and the positive electrode non-coating portion and the negative electrode non-coating portion are positioned at opposite sides to each other.
A negative current collecting plate is attached to the negative electrode non-coating portion, and a positive current collecting plate is attached to the positive electrode non-coating portion. The negative current collecting plate is connected to the case and the positive current collecting plate is connected to the cap assembly to thus draw current to the outside.
When the negative current collecting plate is connected to the case, the case serves as a negative electrode terminal. When the positive current collecting plate is connected to the cap assembly, the cap assembly serves as a positive electrode terminal. The cap assembly and the case are coupled with each other in an insulation structure through a gasket.
The cap assembly includes a cap plate, a positive temperature device, a vent plate, an insulator, a middle plate, a sub-plate, and a connecting member that are sequentially provided from the outside. The connecting member electrically connects the positive current collecting plate and the middle plate. The vent plate and the sub-plate are connected to each other by welding, with the insulator and the middle plate interposed therebetween.
The vent plate forms a vent which is to be connected to the sub-plate, the vent and the sub-plate form a connection portion, and as the connection portion is formed by welding, they are easily broken and disconnected when the internal pressure of the battery rises, to thus cut off current. That is, the current interrupt device (CID) is formed between the vent plate and sub-plate of the cap assembly.
To this end, the vent plate is provided with notches so that the circumference of the vent has a smaller thickness than other portions of the vent plate have. However, such a cap assembly interrupts current while the vent is separated as the notches are ruptured by explosion in the event of an increase of the internal pressure of the battery.
In this manner, the current interrupt device cuts off current after explosion, so that the current interrupt device is not able to properly prevent the explosion of the battery. Moreover, the sub-plate forming the connection portion together with the vent is formed in a plate shape, and hence there is a large dispersion of an operating pressure, i.e., separation pressure, at which the connection portion of the sub-plate and the vent is disconnected.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.